An example of various types of control systems for a three-phase motor is disclosed in U.S. Pat. No. 6,555,988 B2 corresponding to Japanese Patent Application Publication No. 2001-1169590.
A control system disclosed in the US Patent Publication superimposes a high-frequency AC voltage on an output signal from an inverter in an estimated direct axis (d-axis) θ of a rotor of a salient three-phase motor; this inverter supplies AC power thereto. The control system also calculates a rotation angle of the rotor based on a current signal actually created in the AC motor.
The salient structure (salient characteristic) of the three-phase motor means a characteristic in which a magnetic resistance created in a d-axis of the three-phase motor is higher than that created in a q-axis thereof. In other words, the salient structure (salient characteristic) of the three-phase motor means a characteristic in which inductance in the d-axis of the tree-phase motor is lower than that in the q-axis thereof.
Specifically, the three-phase motor has the minimum inductance in the real d-axis direction of the rotor because of the salient structure of the rotor, which allows current to flow easier in the d-axis of the rotor than that following in a quadrature axis (q-axis) thereof. For this reason, the current signal actually created in the three-phase motor has a d-axis component higher than a q-axis component thereof independently of the phase of the superimposed AC voltage.
Thus, when a vector of the current signal actually created in the three-phase motor is deviated from the estimated d-axis, the estimated d-axis is determined to be deviated from the actual d-axis of the rotor. As a result of focusing on the deviation, the control system repeatedly executes the AC voltage superposition so as to reduce the deviation between the estimated d-axis and the actual d-axis for each executions thereby improving the accuracy of estimation of the rotation angle of the rotor.
Increase in an output torque of a three-phase motor may invite partially magnetic saturation in a three-phase motor. This may cause a direction in which the inductance is minimum to be shifted from the actual d-axis toward a vector of a current for driving the three-phase motor, and therefore, the inductance in the d-axis direction becomes substantially equal to that in the q-axis direction. This may make it difficult for the control system disclosed in the US Patent Publication to properly calculate the rotation angle of the rotor.
Especially, in recent years, downsizing in three-phase motors and growing desire for high output torque may encourage magnetic saturation to be created. For this reason, the control system disclosed in the US Patent Publication may be more difficult to properly obtain the rotation angle of the rotor due to the magnetic saturation.